The present invention relates to a bidirectional DC-DC converter that is connected between a first DC (direct current) voltage line and a second DC voltage line and a method of controlling a bidirectional DC-DC converter. The bidirectional DC-DC converter bidirectionally transmits DC power between the first DC voltage line and the second DC voltage line. Note that the first DC voltage line is connected, for example, to an AC-DC converter that generates a DC voltage from a commercial AC (alternating current) power source. The second DC voltage line is connected, for example, to a capacitor (storage battery).
As a conventional bidirectional DC-DC converter, a bidirectional DC-DC converter has been proposed by the applicant as disclosed in Japanese patent publication No. 2010-5266 (See page 6 and FIG. 1). The bidirectional DC-DC converter (DC-DC converter) is configured with a transformer, a voltage-source power converter, a current-source power converter and a controller. Specifically, the voltage-source power converter performs power conversion by controlling a voltage that is applied to a primary side of the transformer (a primary winding). The current-source power converter performs power conversion by controlling a current that flows in a secondary side of the transformer (a secondary winding). The controller controls the voltage-source power converter and the current-source power converter. The bidirectional DC-DC converter can perform a charging operation for a capacitor (storage battery), which is connected to the current-source power converter, by generating a DC voltage based on a rail voltage (DC voltage) that is supplied to a side of the voltage-source power converter and an output operation for providing the rail voltage (DC voltage) by generating the DC voltage based on the DC voltage stored in the capacitor (storage battery).
Specifically, in the above described conventional bidirectional DC-DC converter, when the capacitor (storage battery) is charged, the voltage-source power converter generates an AC voltage by switching the rail voltage with a switching element and outputs the AC voltage to the primary winding of the transformer. As a result, the current-source power converter rectifies an AC voltage that is generated in the secondary winding of the transformer, converts the AC voltage to a DC voltage and at the same time outputs the DC voltage to the capacitor (storage battery). On the other hand, when the capacitor (storage battery) is discharged, the current-source power converter generates an AC current by switching the DC voltage, which is supplied from the capacitor (storage battery), by the switching element and outputs the AC current to the secondary winding of the transformer. As a result, the voltage-source power converter rectifies the AC current that is output from the primary winding of the transformer, converts the AC current to the DC voltage and at the same time outputs the converted DC voltage as the rail voltage.
However, the conventional bidirectional DC-DC converter can be improved. Specifically, in the conventional bidirectional DC-DC converter, even when a charging current that is supplied to the capacitor (storage battery) is low (in other words, a load of the current-source power converter is light), the controller makes the switching elements for the voltage-source power converter and the current-source power converter continuously operate at a predetermined switching cycle that is the same as when the charging current is high. Therefore, the conventional bidirectional DC-DC converter is not as efficient as desired because a switching loss (turn-on power loss and turn-off power loss) always occurs for the switching elements.
An object of the present invention is to provide a bidirectional DC-DC converter that can achieve further improved efficiency.